1. Field of the Invention
This invention pertains to multirange hydromechanical transmissions wherein shifts between ranges are accomplished by the relatively simultaneous activation of a brake or clutch controlling operation in one range and the deactivation of the brake or clutch controlling another range. These transmissions can be either one output (truck) or two output (steering) transmissions. Within this category, this invention more specifically pertains to those transmissions having hydraulic components to provide continuously variable drive ratios such as a pump/motor circuit and in which, as for example in an extended range transmission, there are different pressure-to-torque ratios in different ranges or in which for other reasons a discontinuity in output power or efficiency between ranges occurs at a shift point. One type of transmission within the field of invention is that in which a change of operating mode within the hydraulic system occurs at shift points, as for example in some synchronous shift transmissions, since such changes are often accompanied by a temporary or transitory condition in the hydraulic pump/motor circuit or a step to a higher operating pressure.
2. Description of the Prior Art
In addition to my U.S. Pat. Nos. 3,815,698 and 3,861,240; the patents to Mooney, U.S. Pat. Nos. 3,714,845; Orshansky, 3,709,061, 3,709,060 and 3,580,107; Polak, 3,596,535; and DeLalio, 3,306,129 are exemplary of systems in which successive ranges operate with different pressure-to-torque ratios, or in which an upshift in range is accompanied by a step to a higher hydraulic operating pressure, or in which there is a discontinuity of output power or efficiency at one or more shift points. FIGS. 7 and 8 of Orshansky, U.S. Pat. No. 3,709,060; FIGS. 6 and 13 of DeLalio, U.S. Pat. No. 3,306,129; FIGS. 19, 20, 30 and 31 of Orshansky, U.S. Pat. No. 3,580,107; FIG. 2 of Mooney, U.S. Pat. No. 3,714,845; and FIG. 2 of Polak, U.S. Pat. No. 3,596,535 are indicia of such criteria, either because of a change of slope of component speeds compared to vehicle speed or because of indicated changes in hydraulic operating pressures. In each of these systems, it is likely that when the transmission is working under a load that passes through the discontinuity in efficiencies, i.e., a load that would offer a resistance which would not impede acceleration of the vehicle in the lower of two ranges but is sufficient to preclude acceleration in the next higher range, or for that matter cause deceleration in the upper range, the load would cause the transmission to "hunt", i.e., to constantly shift back and forth between the two gear ranges. Although not particularly pointed out in any of the patents cited, it has been customary in the art to attempt to overcome this "hunting" problem by means of the use of time delays, hysteresis valves, and other control techniques. The configuration of many of these transmissions are also such that operation in the higher of the ranges at or near the shift point may cause one or more elements to assume a motion which is exactly that which the brake or clutch of the lower range is designed to prevent. Such motion can be self-defeating for the system, particularly in those instances in which a reaction point is made unavailable, as for example a ring gear in a planetary set which backs up instead of varying from zero to a high forward speed.
None of the art with which I am familiar solves this problem in the manner which constitutes the subject matter of this application. Although Phillips in U.S. Pat. No. 3,597,997 uses a one-way brake 41 and one-way clutch 60, neither the situation nor the result of such usage corresponds to either the problem or to my solution. Phillips was apparently concerned with the improvement of the quality of shifts and devised his system to avoid very stringent requirements of brake and clutch timing. More details pertaining to the prior art will be apparent from my description of a preferred implementation as applied to one sample transmission within the class defined.